1. Field of the Invention
The present invention relates to a method of specifying a Cu-contamination-causative step (or steps) that introduced Cu into a silicon wafer (hereinafter, referred to as “Si wafer”), particularly, a test Si wafer, i.e., a Si wafer used for monitoring a semiconductor chip manufacturing process or for setting operating conditions for a semiconductor device fabricating system, during a reclamation process, a method of detecting Cu-contamination, and a method of reclaiming a Si wafer. Those methods of the present invention are capable of simply, accurately, nondestructively detecting Cu contaminating the wafer during a reclamation process for reclaiming Si wafers coated with metal films including Cu films, and are used effectively for the reclamation of used Si wafers subjected once to a semiconductor device fabricating process.
2. Description of the Related Art
Generally, a method of reclaiming a used Si wafer includes a film-removing process of removing a film formed on a surface of a wafer, a polishing process of polishing the surface of the wafer in a mirror-finished surface, a cleaning process of cleaning the polished wafer, and a quality evaluation process of evaluating the reclaimed wafer. The film-removing process is a process specific to wafer reclamation. Since the condition of a wafer obtained by removing a film from a used wafer is the same as that of a virgin wafer, the reclaimed wafer can be processed by the following processes including the polishing process and the cleaning process, which are the same as those for processing a virgin wafer.
Various Si wafer reclamation methods including an improved film-removing process have been proposed. A film-removing process disclosed in Reference patent document 1 (U.S. Pat. No. 5,855,735) removes a film by forming shallow microcracks in the film by using a polishing liquid containing abrasive grains, and a rotary pad (Refer to claims). A film-removing method of removing metal films, silicon oxide films and silicon nitride films disclosed in Reference patent document 2 (U.S. Pat. No. 3,923,567) uses an etching process using an acid (Refer to claims). A film-removing method disclosed in Reference patent document 3 (JP-A No. 17833/1997) measures an infrared absorption spectrum indicating infrared absorption by a used wafer prior to a film-removing process to estimate the type of the film formed on the used wafer, and removes the film by a suitable etching process (Refer to claims).
Metal concentration on the wafer surface can be reduced by removing the film by these methods. Therefore those methods are effective in reclaiming used wafers covered with films containing metals used for fabricating semiconductor devices, such as Al, Ti and W. A Si wafer reclamation process processes used wafers covered with films containing various metals, such as metal films, metal silicide films, metal oxide films, metal nitride films and such. It has been known that, although it is possible that some metals, such as Al, Ti and W, removed from used wafers adhere again to the surfaces of the wafers in the wafer reclamation process, the metals once removed from the wafers do not permeate the wafers. Thus, it has been possible to reduce metal concentration on the surface only by the aforesaid film-removing method.
Recently, Cu has become used prevalently as a wiring material instead of Al which has been widely used as a wiring material. Copper, as compared with Al, has satisfactory electrical conductivity, has high resistance against degradation by electromigration, and is more suitable as a material for forming wiring lines than Al. However, since Cu, as compared with other transition metals, has a very large diffusion coefficient in silicon, Cu not only adheres to the surface of a wafer but also penetrates and diffuses easily in the wafer. Moreover, it is suspected that contamination with Cu spreads over the stages of the reclamation process and other products are contaminated with Cu. It is difficult to remove Cu permeated the wafer even by the aforesaid film-removing method.
With a view to reclaiming a Si wafer provided with a Cu film, a method of removing a Cu film disclosed in Reference patent document 4 (JP-A No. 158207/2002) dissolves the Cu film with a special etching chemical solution (Refer to claims). This method, however, needs the special etching chemical solution and can be applied only to reclaimable wafers provided with a special film, such as a barrier film for stopping Cu diffusion or a SiO2 film, before using the same as test wafers.
A method of reclaiming Si wafers provided with a metal film, such as a Cu film, disclosed in Reference patent document 5 (JP-A No. 164558/2000) removes a metal film by chemical etching using an alkali solution or an acid solution (Refer to claims). This method, however, is intended to remove metallic contaminants in the surface of an oxide film underlying a metal film and has difficulty in detecting and removing Cu permeated a Si wafer.
A nondestructive Cu content measuring method of measuring the Cu content of a Si wafer is disclosed in Reference patent document 6 (JP-A No. 64113/1997), however it is not intended for detecting contamination with Cu in a Si wafer reclamation process. This method includes the steps of heating a semiconductor wafer at a temperature not higher than 600° C., and measuring Cu concentration on the surface of the semiconductor wafer (Refer to claims). This method uses the effect of heating the semiconductor substrate on gathering Cu diffused in the semiconductor wafer on the surface of the Si wafer. This method, according to description provided in the specification, heats a Si wafer at a high temperature of about 500° C. The inventors of the present invention found through studies that the quality of a Si wafer is deteriorated when the Si wafer is heat-treated at such a high temperature. Thus, it is improper to apply this method to the reclamation of used Si wafers.
It is reported in Reference literature 1 (Helene Prigge, Peter Gerlach, Peter O. Hahn, Anton Schnegg, and Herbert Jacob, “TECHNICAL PAPERS SOLID-STATE SCIENCE AND TECHNOLOGY”, J. Electrochem. Soc., 1991, Vol. 138, No. 5, 1385-1389) that a wafer polished by using a polishing slurry containing amine or ammonia as an alkali component undergoes bulk contamination with Cu if Cu is mixed into the polishing slurry and, consequently, the electrical conductivity of p-type Si wafers changes (Refer to ABSTRACT, P. 1385). However, studies mentioned in Reference literature 1 was made to elucidate a phenomenon that causes chemical-mechanical polishing to change the resistivity of a Si wafer, and merely found that the contamination of the Si wafer with Cu in the polishing process was the cause of the phenomenon, which is different from the conventional idea that the change of the resistivity of a Si wafer is due to the inactivation of the dopant by hydrogen atoms and structural defects caused by polishing. Thus, the studies mentioned in Reference literature 1 are not intended at all to specify a Cu-contamination-causative step in the Si wafer reclamation process, for which the present invention is intended.